linux-kernel - Re: [PATCH v3 1/2] x86_64,signal: Fix SS handling for signals delivered to 64-bit programs

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Date:	Wed, 18 Mar 2015 15:03:27 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Pavel Emelyanov <xemul@...allels.com>
Cc:	Oleg Nesterov <oleg@...hat.com>, Andrey Wagin <avagin@...il.com>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Andy Lutomirski <luto@...nel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andi Kleen <andi@...stfloor.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Al Viro <viro@...iv.linux.org.uk>, X86 ML <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Pavel Emelyanov <xemul@...nvz.org>
Subject: Re: [PATCH v3 1/2] x86_64,signal: Fix SS handling for signals
 delivered to 64-bit programs

On Wed, Mar 18, 2015 at 2:34 PM, Pavel Emelyanov <xemul@...allels.com> wrote:
> On 03/19/2015 12:26 AM, Andy Lutomirski wrote:
>> On Wed, Mar 18, 2015 at 1:02 PM, Oleg Nesterov <oleg@...hat.com> wrote:
>>> On 03/18, Andrey Wagin wrote:
>>>>
>>>> This patch fixes the problem. Oleg, could you send this path in the
>>>> criu maillist?
>>>
>>> Sure, will do.
>>
>> We still haven't answered one question: what's the kernel's position
>> on ABI stability wrt CRIU?  We clearly shouldn't make changes that
>> break the principle of CRIU, but CRIU encodes so many tricky
>> assumptions about the inner workings of the kernel that it's really
>> tough to avoid breaking old CRIU versions.
>
> Well, we try hard to use only documented kernel API-s. Isn't the sigframe
> considered to be some sort of "stable API"? I mean -- it's visible by the
> userspace, nobody prevents glibc or gdb from messing with this stuff just
> by reading it from memory.
>
> If it's "parse-able" e.g. like VDSO is, but we don't do it in CRIU -- then
> it's definitely a CRIU BUG to be fixed.

It's certainly parseable by things like gdb.  But it's also supposed
to be extensible.  hpa, any thoughts here?

>
>> So... do we introduce somewhat nasty code into the kernel to keep old
>> CRIU versions working, or do we require that users who want to restore
>> onto new kernels use new CRIU?
>
> It's OK (I think) to require newer versions of CRIU, it's easy to update
> one unlike the kernel ;)
>
> But if "old" version of CRIU just crash the restored processes on "new"
> kernels and there's no way to detect this properly -- that's the problem.

Yeah, that's unfortunate.

I don't have a great idea for how to work around this, unfortunately.
Ideally we'd increment some kind of version counter or use an
extension mechanism rather than shoving ss into a field that used to
be padding.

--Andy

>
>> (It seems clear to me that CRIU should apply the patch regardless of
>> what the kernel does.  It will enable CRIU to work on the same class
>> of programs that are fixed by the kernel change that started this
>> thread.)
>>
>> --Andy
>> .
>>
>
> Thanks,
> Pavel
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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